The present invention relates to high frequency filter circuits and more particularly to K inverters which may be utilized in coaxial microwave filters.
A variety of circuits for forming microwave and other high frequency filters are known in the prior art. Generally, it is known that a filter can be theoretically constructed using a combination of series resonators alternating with shunt resonators to produce a particular bandpass design. Practically, however, it is difficult to achieve a microwave structure which physically has the arrangement of alternating series and shunt resonators. Accordingly, an approximation of such a filter is formed using resonators in combination with impedance inverters to produce an effect similar to alternating series and shunt resonators and thereby enable the production of a practical microwave filter. The impedance inverters are generally known as K and J inverters and their effect and construction in exemplary prior known circuits is described in the book entitled "Microwave Filters, Impedance-Matching Networks, and Coupling Structures" by Matthew Young and Jones, McGraw-Hill, 1964, pp. 427-472 and in the article entitled "Quarterwave Dielectric Transmission Line Diplexer for Land-mobile Communications" by Wakino, et.al. in the MTT Symposium 1980, pp. 278-280.
Such filters, as are referred to above, generally include a plurality of two port quarter-wave resonators alternately coupled by K and J inverters to produce the series and shunt resonator effect needed to produce particular configurations of bandpass filters. Such configurations have been designed in various microwave circuits including stripline and coaxial embodiments. While such structures and circuits have enabled the successful construction of various high-frequency filters, there is a continuing need for new filter designs which enable reductions in size and complexity for use in new technology electronic systems. More particularly, in some prior art filter circuits using known resonator filtering techniques, the size of the structures required to form the resonators and K and J inverters does not allow any further reduction in filter size without significantly sacrificing filter operation and efficiency. In still other instances, filter circuits formed using precisely constructed elements of specific configurations require significant increases in the time and manpower employed in the manufacturing processes required to produce acceptable devices. In these instances, without precise manufacturing process control, the resulting filters are less efficient, do not have as wide a range of operation, and suffer from greater losses.
In an effort to overcome the above-mentioned and simiar deficiencies, other configurations have been proposed which simplify the construction of the filter and resonators included therein. One example is illustrated in U.S. Pat. No. 4,255,729 in which a plurality of resonators are enclosed within a conductive housing and coupled to one another to provide the requisite filter characteristics. While the structure attempts to reduce the reliance on precision manufacturing processes while improving coupling and reducing losses, there is a continuing need to provide further simplified structures capable of being applied in a variety of environments and being sufficiently versatile to allow variations in the filter characteristics.
Accordingly, the present invention has been developed to overcome the specific shortcomings of the above known and similar techniques and to provide a simplified and less expensive K inverter and filter circuit for use in high frequency systems.